1. Field of the Invention
The present invention relates to the field of nucleic acid hybridization. More specifically, some embodiments of the present invention relate to methods of locally destabilizing the hybridization between two nucleic acid molecules thereby providing a region of unpaired nucleic acids that is free to interact with other molecules.
2. Description of the Related Art
Hybridization of polynucleotides to other polynucleotides having at least a portion of complementary nucleotide sequence by Watson-Crick base pairing is a fundamental process useful in a wide variety of research, medical, and industrial applications. Detecting the hybridization of a probe to a polynucleotide containing a target sequence is useful for gene expression analysis, DNA sequencing, and genomic analysis. Particular uses include identification of disease-related polynucleotides in diagnostic assays, screening for novel target polynucleotides in a sample, identification of specific target polynucleotides in mixtures of polynucleotides, identification of variant sequences, genotyping, amplification of specific target polynucleotides, and therapeutic blocking of inappropriately expressed genes, e.g. as described in Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd Edition (Cold Spring Harbor Laboratory, New York, 1989); Keller and Manak, DNA Probes, 2nd Edition (Stockton Press, New York, 1993); Milligan et al., 1993, J Med Chem, 36: 1923-1937; Drmanac et al., 1993. Science, 260: 1649-1652; Bains, 1993, J DNA Seq Map, 4: 143-150.
Immobilized probes are useful for detecting polynucleotides containing a target nucleotide sequence, where each immobilized probe is functionally connected to a support and the hybridization of a polynucleotide to the immobilized probe can be detected. Most commonly, DNA probes are used to detect polynucleotides containing a target nucleotide sequence complementary to the probe sequence. The support for immobilized probes may be a flat surface, often called a “chip,” or the support may be the surface of a bead or other particle. Probes are usually immobilized in a known arrangement, or array, which provides a medium for matching known and unknown polynucleotides based on base-pairing rules. Preferably, the process of identifying the unknowns identified using a probe array is automated. Microarrays having a large number of immobilized probes of known identity are used to determine complementary binding, allowing massively parallel studies of gene expression and gene discovery. For example, an experiment with a single DNA chip can provide researchers information on thousands of genes simultaneously. For example, Hashimoto et al. disclose an array of immobilized single-stranded probes wherein at least one probe has a nucleotide sequence complementary to the target gene(s) to be detected, such that each probe is immobilized onto the surface of an electrode or the tip of an optical fiber and an electrochemically or optically active substance capable of binding to double-stranded nucleic acid is used to detect hybridization of target genes to complementary immobilized probes (U.S. Pat. Nos. 5,776,672 and 5,972,692).
Universal Chips
Under some circumstances, a drawback to chip technology is that each chip must be manufactured specifically for the sequences to be detected, with a set of immobilized probes that are designed to be complementary to specific sequences to be detected. Chips specific for a single organism require a large manufacturing investment, and the chips can only be used for a narrowly defined range of samples. In contrast, a “universal chip” or “universal array” is organism-independent because the probes are not targeted to organism-specific sequences or products. Chips specific for a specific tissue, physiological condition, or developmental stage, often used for gene expression analysis, can likewise require a substantial manufacturing investment for use with a limited range of samples. A universal chip provides an unrestricted approach to studying tissues, physiological conditions, or developmental stages of interest. Manufacturing quality control can be improved by using a universal chip for polynucleotide detection.
One approach to universal chip design involves attaching a set of oligonucleotide probes to a chip surface, where the set of oligonucleotide probes includes all possible sequences of oligonucleotides that are 5, 6, 7, 8, 9, 10 or more nucleotides in length. The probes needed for these arrays can be designed using a simple combinatorial algorithm. The chip is incubated with a mixture that may contain DNA, cDNA, RNA or other hybridizable material, and hybridization to each probe of known sequence is measured. However, the specificity of such an array may be impaired because different sequences may have different requirements for stringent hybridization. In addition, such a universal array does not prevent false positives resulting from frameshifting where, for example in a universal array having probes that are six nucleotides long, the final four nucleotides of a sample polynucleotide may hybridize to the complementary final four nucleotides of a six-nucleotide probe, but the same sample polynucleotide would not hybridize to the entire six-nucleotide probe sequence.
Suyama et al. (2000, Curr Comp Mol Biol 7:12-13) disclose a universal chip system for gene expression profiling of a sample, where the chip system utilizes “DNA computing” instead of binding of transcripts to probes. The DNA computing system of Suyama et al. indirectly determines which transcripts are present by measuring binding of coded adapters to a universal set of immobilized probes on the universal chip. Only those coded adapters with a region complementary to a region of a transcript present in a sample will undergo the subsequent manipulations and the processing steps that generate adapters capable of binding to probes on the universal chip.
Tags
An alternative approach to manufacturing a universal chip involves using a set of tag sequences that do not naturally occur in the target polynucleotides, where the tags bind to complementary probes on a universal chip. Tags for such uses are sometimes known as “address tags” or “zip codes” or are considered to be analogous to “bar codes” for identifying targets. Detection, identification, tracking, sorting, retrieving or other manipulations are then directed at tag sequences and not the sequences of the target polynucleotides. Oligonucleotide tags may be covalently attached to or incorporated into polynucleotides. Tags may become associated with a polynucleotide by hybridization of a separate oligonucleotide which functions as a linker by virtue of having at least two domains, one with a tag sequence complementary to a probe and one with sequence complementary to at least a portion of the target polynucleotide. Systems employing oligonucleotide tags have been proposed as means for manipulating and identifying individual molecules in complex mixtures, for example to detect polynucleotides having target nucleotide sequences, or as an aid to screening genomic, cDNA, or combinatorial libraries for drug candidates. Brenner and Lerner, 1992, Proc Natl Acad Sci, 89: 5381-5383; Alper, 1994, Science, 264: 1399-1401; Needels et al., 1993, Proc Nat Acad Sci, 90: 10700-10704.
Many applications of chip technology involve PCR amplicons or other nucleic acids that are normally double stranded. A serious disadvantage exists, however, when using such double-stranded nucleic acids in certain chip-based applications. For example, when hybridizing one strand of a double-stranded nucleic acid to a surface, such hybridization must compete with rehybridization with the complementary strand in solution. One solution to this problem has been the use of “asymmetric PCR” to generate single -stranded nucleic acids. In “asymmetric PCR” the template nucleic acid is mixed with an excess of one primer over the other, thereby generating more of the desired strand than the undesired strand. This approach, however, limits the yield of the amplicon which becomes particularly acute in multiplexed applications. Another approach to ameliorating the competition problem is selective exonuclease digestion of the undesired strand. In these applications, however, the desired strand must be made exonuclease resistant. Generating these exonuclease resistant nucleic acids strands requires a more complex synthesis. Furthermore, long incubation times are required for exonuclease digestions. Accordingly, there exists a need for a fast simple method for reducing nucleic acid strand rehybridization that is sufficiently robust for use in multiplexing applications.